安全视频4Linux (v4l) 绑定

此crate提供了对Video for Linux (V4L)堆栈的安全绑定。现代设备驱动程序通常会实现v4l2API，而较旧的驱动程序可能依赖于传统的v4lAPI。可以通过选择此crate的libv4l功能来使用此crate来使用此类传统设备。

目标

此crate应提供v4l-sys包，以启用对libv4l*的完全（但不安全）访问。在此基础上，还将提供更高级、更符合习惯的API来在Linux中使用视频捕获设备。

将提供简单的实用程序应用程序来列出设备和捕获帧。计划未来提供一个最小化的OpenGL/Vulkan查看器来显示帧。

变更日志

请参阅CHANGELOG.md

依赖项

您可以选择两种依赖项（均由此crate内部提供）

• libv4l-sys

  链接到包括libv4l1、libv4l2、libv4lconvert在内的libv4l*堆栈。这具有通过libv4lconvert等在用户空间中模拟常见捕获格式（如RGB3）的优点。但是，libv4l可能不支持某些功能，如userptr缓冲区。

• v4l2-sys

  仅使用Linux内核提供的由videodev2.h提供的v4l2 API。您将获得所有v4l2功能的支持，例如userptr缓冲区，但可能需要自行进行格式转换，例如需要RGB/BGR缓冲区，而此类缓冲区可能不受如网络摄像头等通用设备的支持。

通过选择作为此crate功能的libv4l或v4l2后端来启用。

使用方法

以下提供了此crate的快速使用示例。它介绍了从流设备（例如网络摄像头）捕获帧所需的基本知识。

use v4l::buffer::Type;
use v4l::io::mmap::Stream;
use v4l::io::traits::CaptureStream;
use v4l::video::Capture;
use v4l::Device;
use v4l::FourCC;

fn main() {
    // Create a new capture device with a few extra parameters
    let mut dev = Device::new(0).expect("Failed to open device");

    // Let's say we want to explicitly request another format
    let mut fmt = dev.format().expect("Failed to read format");
    fmt.width = 1280;
    fmt.height = 720;
    fmt.fourcc = FourCC::new(b"YUYV");
    let fmt = dev.set_format(&fmt).expect("Failed to write format");

    // The actual format chosen by the device driver may differ from what we
    // requested! Print it out to get an idea of what is actually used now.
    println!("Format in use:\n{}", fmt);

    // Now we'd like to capture some frames!
    // First, we need to create a stream to read buffers from. We choose a
    // mapped buffer stream, which uses mmap to directly access the device
    // frame buffer. No buffers are copied nor allocated, so this is actually
    // a zero-copy operation.

    // To achieve the best possible performance, you may want to use a
    // UserBufferStream instance, but this is not supported on all devices,
    // so we stick to the mapped case for this example.
    // Please refer to the rustdoc docs for a more detailed explanation about
    // buffer transfers.

    // Create the stream, which will internally 'allocate' (as in map) the
    // number of requested buffers for us.
    let mut stream = Stream::with_buffers(&mut dev, Type::VideoCapture, 4)
        .expect("Failed to create buffer stream");

    // At this point, the stream is ready and all buffers are setup.
    // We can now read frames (represented as buffers) by iterating through
    // the stream. Once an error condition occurs, the iterator will return
    // None.
    loop {
        let (buf, meta) = stream.next().unwrap();
        println!(
            "Buffer size: {}, seq: {}, timestamp: {}",
            buf.len(),
            meta.sequence,
            meta.timestamp
        );

        // To process the captured data, you can pass it somewhere else.
        // If you want to modify the data or extend its lifetime, you have to
        // copy it. This is a best-effort tradeoff solution that allows for
        // zero-copy readers while enforcing a full clone of the data for
        // writers.
    }
}

请查看提供的示例以获取更多示例应用。